As increasingly advanced semiconductor fabrication processes are used these days, precise tuning of various circuit characteristics is an important consideration. Resistance variation is a challenging issue that can make it difficult to design circuitry that meets various performance requirements. After tapeout for a 28 nm fabrication process, variation of up to 30% in resistance has been observed. Such resistance variation is sought to be calibrated to ensure proper circuit performance.
One prior digital approach for calibrating a resistance involved selectively enabling or disabling various combinations of switches coupled to a resistor network. A disadvantage with that approach is that the use of numerous switches necessitates a large circuit area and a large number of external control pins, increasing circuit complexity and cost.